Field of the Invention
The present invention relates to a current integrator and an organic light-emitting display comprising the same.
Discussion of the Related Art
An active-matrix organic light-emitting display includes self-luminous organic light emitting diodes (hereinafter, “OLEDs”), and has the advantages of fast response time, high luminous efficiency, high luminance, and wide viewing angle.
An OLED, which is a self-luminous element, includes an anode, a cathode and organic compound layers HIL, HTL, EML, ETL, and EIL formed between the anode and the cathode. The organic compound layers comprise a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When an operating voltage is applied to the anode and the cathode, a hole passing through the hole transport layer HTL and an electron passing through the electron transport layer ETL move to the emission layer EML, forming an exciton. As a result, the emission layer EML generates visible light.
An organic light-emitting display has pixels arranged in a matrix, each pixel comprising an OLED, and adjusts the brightness of the pixels according to the grayscale of video data. Each pixel includes a driving element—i.e., driving TFT (thin film transistor)—that controls driving current flowing through the OLED in accordance with a voltage Vgs applied between its gate electrode and source electrode. The electrical characteristics of the driving TFTs, such as threshold voltage, mobility, etc., deteriorate with the passage of operation time and may vary from pixel to pixel. Such variations in the electrical characteristics of the driving TFTs cause differences in brightness between the pixels, thus making it difficult to realize a desired image.
As a way to compensate for variations in the electrical characteristics of the driving TFTs, internal compensation and external compensation are well known. In internal compensation, variations in threshold voltage between the driving TFTs are automatically compensated for within a pixel circuit. For internal compensation, the driving current flowing through the OLED should be determined regardless of the threshold voltage of the driving TFT, which makes the configuration of the pixel circuit quite complicated. Moreover, internal compensation is not suitable for compensating for variations in mobility between the driving TFTs.
In external compensation, sensed voltages and currents that match the electrical characteristics (threshold voltage and mobility) of the driving TFTs are measured, and an external circuit connected to a display panel modulates video data based on these sensed voltages, thereby compensating for variations in electrical characteristics. A lot of research is going on today regarding this external compensation approach.
In the conventional external compensation approach, a data driver circuit receives a sensed voltage directly from each pixel through a sensing line, converts this sensed voltage to a digital sensed value, and then feeds it to a timing controller. The timing controller compensates for variations in the electrical characteristics of the driving TFTs by modulating digital video data based on the digital sensed value.
The driving TFTs are current elements, so their electrical characteristics are accounted for by the amount of electrical current Ids flowing between the drain and source in response to a certain gate-source voltage Vgs.
The data driver circuit for the external compensation approach includes a sensing part that senses the electrical characteristics of the driving TFTs. The sensing part includes an integrator made up of an amplifier AMP, an integrating capacitor CFb, and a switch SW. In the integrator, the amplifier AMP includes an inverting input terminal (−) that receives the source-drain current Ids of the driving TFTs, a non-inverting input terminal (+) that receives a reference voltage Vref, and an output terminal that produces an integral, the integrating capacitor Cfb is connected between the non-inverting input terminal (−) and output terminal of the amplifier AMP, and the switch SW is connected to both ends of the integrating capacitor Cfb.
Each of a plurality of amplifiers AMP corresponding to a plurality of sensing lines has an offset voltage, and the offset voltage of the amplifier AMP is included in the integral produced from the output terminal of the amplifier AMP. Referring to FIG. 1, each amplifier AMP has a different offset voltage. In FIG. 1, the X-axis indicates the numbers of a plurality of sensing lines electrically connected respectively to a plurality of amplifiers AMP, and the V axis indicates the offset voltage which is output for each sensing line.
Since each amplifier AMP has a different offset voltage, the integral produced from their output terminal changes with the offset voltage even if substantially the same amount of current is input into the input terminal of each amplifier AMP. The integral has a large degree of dispersion due to the differences in offset voltage between the amplifiers AMP. Referring to FIG. 2, the large degree of dispersion of values of the integral makes it difficult to obtain accurate sensed values. In FIG. 2, the X-axis indicates the output voltage for each sensing line, which is sensed based on the integral, and the Y-axis indicates frequency.
There is a large dispersion among values of the sensed voltage around −50 and 50. When compensating for variations in the electrical characteristics of the pixels by using the sensed voltage values, there may be problems with the compensation characteristics regarding pixel compensation.